The present invention is related to a system and method for solar electric power generation. In particular, the invention is related to a nonimaging optical mixer for producing a substantially uniform irradiance on a concentrating photovoltaic (CPV) array.
Concentrating photovoltaic (CPV) converters are a promising option for solar electric power generation. Photovoltaic cells placed in highly concentrated sunlight can produce electric power very efficiently, eliminating the need for heat engines operating at high temperatures. However, sunlight concentrated by a typical parabolic dish produces highly non-uniform flux in the focal zone. Photovoltaic cells placed in a rectangular array near the focus are subject to this highly non-uniform irradiance, a condition that degrades the electrical output of the cell array. Typical CPV array electrical interconnection schemes result in performance output that is severely limited by the cell with the lowest irradiance. To increase power output, the irradiance across the CPV array must be much more uniform then heretofore achievable.
It is therefore an object of the invention to provide an improved method, system and apparatus for solar electric power generation.
It is another object of the invention to provide a novel method, system and apparatus for solar electric power generation from a concentrating photovoltaic (CPV) array.
It is a further object of the invention to provide an improved method, system and apparatus utilizing an optical mixer to generate a substantially uniform illumination output for enhanced CPV efficiency.
It is an additional object of the invention to provide an improved method, system and apparatus utilizing a highly efficient, inexpensive optical mixer to output a substantially uniform light pattern onto a photoelectric or photovoltaic cell array.
It is yet another object of the invention to provide an improved method, system and apparatus using broken symmetry optical mixers and/or an exit aperture concentrator to provide highly concentrated, substantially uniform output light flux for illumination of a photoelectric cell array.
Other objects, advantages and alternative forms of the invention will become apparent from consideration of the detailed description and drawings described hereinafter.
There are two principal pathways to efficient photovoltaic conversion of solar energy to electricity, namely, low cost cells operated at ambient solar flux and higher cost cells operated with concentrated solar flux. The latter application imposes a requirement on the concentrator that is specific to photovoltaic applications, namely, the irradiance on the cell needs to be quite uniform. This is because non-uniform irradiance on the cell degrades the electrical performance thereby reducing conversion efficiency.
The invention is therefore generally directed to nonimaging optical mixer designs that produce uniform flux for use with photovoltaic dish concentrators. In one example design a reflector tube, such as one having a square cross-section, is placed near the focal zone and preferably just above the focal point of a primary dish concentrator so that entering rays of light are reflected and xe2x80x9cmixedxe2x80x9d until the light exits at the other end of the reflector tube. The focal zone of a concentrator is the area where the light is substantially most concentrated, sometimes referred to as the xe2x80x9ccircle of least confusionxe2x80x9d, or the xe2x80x9cbeam waistxe2x80x9d. A CPV cell array is placed near the exit of the reflector tube where the xe2x80x9cmixedxe2x80x9d light is highly uniform. A second example design uses refractive optics wherein a solid body, such as a fused silica tube, is placed just above or near the focal point of the dish concentrator. The square aperture of the silica tube face closest to the primary is the target plane for the primary dish. Rays of sunlight are refracted as they pass into the silica tube, and only a small amount of energy is reflected and lost. Nearly all the rest of the light that enters the silica tube will reach the exit aperture of the tube with virtually no optical loss. This advantageous result occurs because as light rays pass through the silica tube and encounter the walls of the tube, the rays are entirely reflected due to substantially total internal reflection (TIR). Therefore, at the exit aperture of the silica tube, the light has been well mixed and highly uniform irradiance is achieved. In a third example design the solid body of the optical mixer can be a liquid with containment walls of appropriate index of infraction to perform the TIR functionality. In a fourth example design even higher concentration ratios are achieved, up to about 2000 times, by performing further light concentration within an optical mixer system. In one form of this example, the optical mixer has a truncated, pyramided shape and in another form the optical mixer includes at least one of a CPC (compound parabolic concentrator) disposed at the exit aperture of the selected optical mixer before the CPV cell array. Here we use the term CPC in the generalized sense that has been adopted in the art of nonimaging optics. It includes designing for specific attributes such as the well known concepts of total internal reflection and tailoring for specific angular properties. For example, see Welford and Winston, High Collection Nonimaging Optics, Academic Press, 1989 and U.S. Pat. Nos. 5,586,013 and 5,971,551.